A flat strain wave gearing, as disclosed in Patent Document 1, is provided with two rigid internally toothed gears, one cylindrical flexible externally toothed gear, and one wave generator. The wave generator is provided with an ellipsoidally contoured rigid plug, and a wave bearing press-fitted onto the ellipsoidal external peripheral surface of the plug.
A movement-restricting mechanism that restricts movement of the wave generator in an axial direction is attached to the wave generator. The movement-restricting mechanism has members that protrude in the axial direction from the both sides of the plug of the wave generator, and the members are either supported by bearings or brought into contact with plates, the bearings or plates being disposed on both sides of the wave generator, whereby the plug of the wave generator is restricted from moving in the axial direction.
A known example of a wave generator of a flat strain wave gearing is one that is provided with two wave bearings, which is disclosed in Non-Patent Document 1. The two wave bearings are press-fitted onto the ellipsoidal external peripheral surface of the plug. The use of two wave bearings makes it possible for an externally toothed gear to reliably mesh with each of two internally toothed gears.
In the wave generator 100 disclosed in Non-Patent Document 1, two wave bearings 113, 114 are fixed by press-fitting to an external peripheral surface 112 of a plug 111, as shown in FIG. 5. A retainer restraint 124 is placed between these wave bearings 113, 114, and ball retainers 117, 121 are prevented from separating from both wave bearings 113, 114 in the axial direction. The retainer restraint 124 is a ring-shaped member having a center hole slightly larger than the external peripheral surface 112 of the plug 111, and the retainer restraint 124 faces the plug 111 and the two wave bearings 113, 114 with some clearance.